Static inverters are often used to provide variable voltage and variable frequency power to a load, such as a motor. Such an inverter may be of the polyphase type having three sets of switches which are operated to produce a three-phase output. The inverter switches are typically controlled in a pulse width modulated (PWM) mode of operation wherein the output voltage developed by the inverter is controlled by varying the length of time the inverter switches are turned on. When energizing such a load, it may be necessary to provide a very low magnitude of inverter output voltage. This, in turn, can only be provided in a PWM mode of operation by turning the switches on for short periods of time. However, as is generally known, conventional power switches of the bipolar transistor type cannot be turned on for less than a minimum on time without damage thereto. Thus, the inverter cannot supply less than a particular magnitude of output voltage without running a substantial risk of inverter switch failure.
One example of where this problem arises is when an inverter is controlled to operate a starter/generator as a motor to supply motive starting power to a prime mover. Such components may comprise part of a variable speed, constant frequency (VSCF) generating system used, for example, in an aircraft.
Patents disclosing PWM controls for an inverter include U.S. Pat. Nos. 4,318,167, to Angquist, 4,533,836, to Carpenter et al., 4,364,109 to Okado et al. and 4,559,593, to Glennon, assigned to the assignee of the instant application. None of these patents, however, addresses the problem of operating switches in a PWM inverter to produce a low voltage output without risking damage to or destruction of the switches.